HAZARD DETERMINATION FOR RARE METEOROLOGICAL

FOR RARE METEOROLOGICAL PHENOMENA

INTRODUCTION

5.1. This section describes methods for establishing the hazard for rare meteorological phenomena such as tornadoes or waterspouts, tropical cyclones and

other events. These events may also result in flooding in certain circumstances. The method can be summarized as follows:

(a) The potential in the region for each phenomenon is assessed. If there is a potential, the regional climatology is evaluated, and the intensity and frequency of occurrence of the phenomenon under consideration are determined.

(b) The relevant physical parameters associated with different intensities of the phenomenon are identified.

(c) The probability of each phenomenon at the specific site is determined as a function of the intensity level of the phenomenon, or an appropriate model for the phenomenon in the region is constructed.

(d) The design basis phenomenon corresponding to a specified probability of exceedance value is evaluated.

TORNADOES

5.2. Tornadoes are generally described as violently rotating columns of air, usually associated with a storm. Waterspouts are similar to tornadoes but they form over large water bodies under more homogeneous surface conditions. If tornadoes or waterspouts strike buildings or structures of a plant, damage may be caused by the following:

(a) The battering effect of very high winds,

(b) The sudden pressure drop which accompanies the passage of the centre of a tornado,

Furthermore, tornadoes may induce floods and consequently may be the cause of additional indirect damage.

Data collection

5.3. Tornado phenomena, identified by appropriate local names, have been documented around the world. Information over as long a period of time as possible should be collected in order to determine whether there is a potential for the occurrence of tornadoes in the region.

5.4. If the possibility that tornadoes may occur in the region is confirmed, a more detailed investigation should be performed to obtain suitable data for the evaluation of a design basis tornado.

5.5. An intensity classification scheme similar to that developed by Fujita–Pearson should be selected. This system is a combination of the Fujita F scale rating for wind speed, the Pearson scale for path length and the Pearson scale for path width. The classification of each tornado is based on the type and extent of damage. Descriptions and photographs of areas of damage provide additional guidance for the classification of the tornado.

Compilation of tornado inventory

5.6. Reports of tornadoes occurring in the region should be collected and the tornadoes should be classified. From this, a regional tornado inventory should be compiled in the form of a ‘tornado catalogue’. A region of the order of 100 000 km² centred at the site should be considered for this purpose.

5.7. Classification of each tornado should include the intensity (F scale), path length, path width and path direction. Information is generally available only for that portion of the occurrence for which the tornado was in contact with the ground. It is difficult to take into account those tornadoes that do not come into contact with the ground at all, or to assign an effective damage for the lifted part of a tornado which touches the ground intermittently. This may result in an underestimate of the probability of interaction with tall structures.

5.8. Correct interpretation of tornado reports collected from the public may be difficult. If the description of a tornado is vague, the F scale intensity class should be assigned conservatively. For the evaluation of the design basis tornado described in this section, the path area (path width and path length) and the intensity (F scale) are very important.

5.9. For the evaluation of the design basis tornado, a region which is climatologically homogeneous and which exhibits uniform tornado characteristics should be selected. The region may be divided into subregions, and for each subregion the frequency of occurrence of tornadoes should be evaluated and compared in order to assess the homogeneity of the zone and the conservatism of the choice of frequency for the region.

Data for design purposes

5.10. The probability per annum that a particular site will experience tornado wind speeds in excess of a specified value should be derived from a study of the tornado inventory. Tornadoes are classified in terms of their physical characteristics, such as maximum wind speed (intensity) and damage area (path length and path width).

5.11. After determination of the design basis tornado, which is scaled by wind speed, a tornado model should be selected in order to evaluate, by the best method available, the other parameters such as the tangential velocity, the maximum rotational wind velocity, the radius to the maximum wind velocity and the pressure drop. Tornado generated projectiles should also be specified.

TROPICAL CYCLONES

5.12. The approach that should be adopted for design measures against tropical cyclones relies on the determination of a probable maximum tropical cyclone (PMTC), which is covered by the present Safety Guide.⁸General methods are given for the evaluation of the relevant parameters of the PMTC. These methods depend on the results of theoretical studies on the tropical cyclone structure and make use of a large amount of data.

5.13. The distribution of heavy rains in tropical cyclones and its estimation and the effects of tropical cyclones on flooding require special consideration. General criteria, not specifically related to tropical cyclones, are presented in the Safety Guide on flood hazards [2].

Description of the phenomenon

5.14. A tropical cyclone consists of a rotating mass of warm humid air, one kilometre to several hundreds of kilometres in diameter. The atmospheric pressure is lower near the centre and could be less than 90 kPa in a well developed severe tropical cyclone. In the northern hemisphere the winds of a cyclone spiral inwards towards the centre in an anticlockwise sense, whereas in the southern hemisphere the rotation is clockwise. Well developed tropical cyclones have widespread areas of thick cloud cover, extending to great heights, together with bands of torrential rain and violent winds. The strongest winds (which may reach 100 m/s) blow in a tight band around the eye of a tropical cyclone.⁹The eye is a region of light winds and lightly clouded sky, usually circular or elliptical in shape and ranging from a

8 It should be borne in mind that, in spite of this accepted terminology, the event is not characterized by purely probabilistic methods.

9 A tropical storm is similar to a tropical cyclone but of lower intensity. A tropical storm corresponds to a maximum wind velocity lower than 33 m/s.

few kilometres to over 150 km in dimension. The wind speeds increase abruptly near the outer edges of the eye, called the eye wall, and then diminish gradually with distance from the wall.

5.15. Although the winds in a tropical cyclone frequently exceed 50 m/s, the cyclone’s translational movement is much slower. For example, in the northwest Pacific ocean, the movement would typically be towards the west or northwest at about 4–5 m/s, but other directions and speeds up to and above 15 m/s are not uncommon.

5.16. The physical processes and energy transformations occurring in tropical cyclones are extremely complex and are not yet fully understood. Essentially, a tropical cyclone is a vast heat engine whose source of energy is the warm sea, providing water vapour which releases latent heat when it condenses and forms rain.

5.17. Tropical cyclones are warm core storms. Since the warm air in the core is lighter than its surroundings, the surface pressure there is lower, and such differences in the surface pressure produce the familiar pattern of circular isobars. Air starting to move towards the low pressure centre is deflected because of the rotation of the earth and spirals inwards. It should be noted that tropical cyclones do not form near the equator (5^°N latitude to 5^°S latitude).

5.18. It is generally known that for a tropical cyclone to form and persist, three conditions must be fulfilled:

(1) The sea must be warm, with a surface temperature of over 27^°C.

(2) Moist air at low levels must converge inwards over a large area.

(3) The air flow at very high levels must be outwards so that circulation can be sustained.

5.19. Tropical cyclones have various names, depending on their severity and the regions in which they occur. What in the Atlantic is described as a hurricane is essentially the same phenomenon as what in the Bay of Bengal, the Arabian Sea and the southwest Indian Ocean is called a severe cyclone or in the western north Pacific a typhoon.

5.20. Although tropical cyclones occur much more rarely than severe EPSs, their impact is sufficiently important to most States concerned to merit a continual reassessment of their threat to coastal areas. The major damage from these storms results from inundations by tide surges accompanying the disturbances and generally occurs some distance away from the centres of the cyclones. On exposed

shorelines, destruction normally begins with erosive scouring and battering by large breaking waves, the effects of which may extend inland with a rising tide to attack a plant’s foundations and to cause structural damage to the lower floors of buildings.

5.21. In general, tropical cyclones occur most frequently in the western Pacific. They also form in the north Indian Ocean (Bay of Bengal and Arabian Sea), the south Indian Ocean, the south Pacific, the west Atlantic and off the northwest coast of Australia.

Tropical cyclones are also frequent in the eastern Pacific but their trajectories remain mainly over the ocean. The occurrence of cyclones is strongly modulated by the southern oscillations: more in the Pacific, fewer in the Atlantic, in El Niño years. This phenomenon is related to the occurrence, every few years, of unusually warm ocean conditions along the tropical west coast of South America, which affect the local weather and create far field anomalies in the equatorial Pacific, Asia and North America. The southeast Atlantic and the central Pacific are not affected by these disturbances. Coastal areas of Brazil are reported to have been subjected to tropical cyclones roughly once every hundred years. There are indications of a steady increase in the temperature of surface water in the oceans, which may theoretically result in an increase in both the rate of occurrence and the intensity of tropical cyclones around the world.

Collection of information

5.22. In view of the available data as a whole, it may be said that a great deal is known about the characteristics of the movement of tropical cyclones and their effects on land and sea, but meteorological measurements at the surface and in the upper air in tropical cyclones are still inadequate in terms of either area coverage or record period.

5.23. As stated, studies of tropical cyclones have generally been handicapped by a lack of data. Early developments in establishing international observation networks have been slow and stations on islands in oceans are few and far between. Tropical cyclones form and exist mostly over oceans, and it is a particularly difficult task to obtain sufficient data to enable a detailed analysis to be made of their thermal and dynamic features. When a tropical cyclone moves over land, it is usually in a weakening stage, and observations even from a relatively dense land observation network may not be representative of the characteristics of an intensifying or intense steady state tropical cyclone.

5.24. In recent years, high resolution images from orbiting and geostationary meteorological satellites have become readily available to many national meteorological services. Such images provide valuable information for the detection and tracking of tropical disturbances, the estimation of their intensity and the derivation of the wind field at cloud level. Nevertheless, the number of parameters for

tropical cyclones that can be measured accurately is still too low to permit reliable descriptions to be given of the basic physical processes involved.

5.25. Reports from reconnaissance aircraft provide important information about tropical cyclones. Data from such reports have been used extensively, in conjunction with conventional synoptic data and autographic records, to throw light on the three dimensional structure of the core regions of tropical cyclones. Observations by aircraft reconnaissance for intense tropical cyclones are carried out near the coasts of Japan, China (Taiwan) and the Philippines, while detailed analyses are made of all the extreme storms along the Gulf of Mexico and the east coast of the United States of America.

5.26. The following data on the storm parameters for tropical cyclones should be collected:

— minimum central pressure;

— maximum wind speed;

— horizontal surface wind profile;

— shape and size of the eye;

— vertical temperature and humidity profiles within the eye;

— characteristics of the tropopause over the eye;

— positions of the tropical cyclone at regular, preferably six hourly, intervals;

— sea surface temperature.

5.27. Values of some of these parameters are generally available in published reports and from databases, summaries or papers by national or international meteorological services or by research institutes. However, some of the data may not be available for a specific region, and recourse should be made to other sources such as radar observations, satellite imagery, special reconnaissance reports, case studies and press reports.

5.28. For the determination of the ‘extreme’ values of some of the variables, the

‘highest’ and ‘lowest’ values that have been recorded should be ascertained. Since synoptic observations are made at discrete time intervals, some of these values may be determined by the use of autographic records from land based locations or ships at sea. If autographic data are insufficient, data on some parameters, such as the maximum winds or the peripheral pressure of a tropical cyclone, should be estimated from synoptic maps.

5.29. For the purposes of applying certain methods, an overall picture should be obtained of the normal or ‘undisturbed’ conditions prevailing in the region when a cyclone occurs. To this end, climatological charts or analyses depicting the following fields should be examined:

— sea level pressure;

— sea surface temperature;

— temperature, height and moisture (dew points) at standard pressure levels and at the tropopause.

5.30. Most of the tropical cyclone data used for the development of the PMTC are associated with storms over open waters and, strictly speaking, the methods are only applicable to open coastal sites. For inland locations, the effects of topography and ground friction should be examined and quantified. In addition, it is known that polewards moving storms generally lose their quasi-symmetrical tropical characteristics and assume the structure of EPSs with well marked thermal contrasts.

In considering the site evaluation for facilities at higher latitudes, modifications should therefore be made to the criteria developed for coastal sites.

Cyclone modelling

5.31. In spite of the availability of aircraft reconnaissance data accumulated over the past 20 years, the time variations of a few of the pertinent tropical cyclone parameters over a period of a few hours are still little known, so the PMTC is assumed to be in a steady state. Substantial changes in the inner core region from hour to hour have been noted in some mature tropical cyclones.

5.32. In order to determine the applicability of a model for a particular site, the local conditions, the peculiarities of the site and the historical data should be carefully evaluated and should be supplemented by means of measurements made with suitable instrumentation installed at the site so that comparisons with surrounding areas may be made. Whenever possible, case studies should also be made in order to determine the characteristics of tropical cyclones that have traversed the vicinity. All known tropical cyclones that have passed within 300–400 km of the site should be included in the study.

5.33. It is possible that the methods based on a physical model for cyclones developed for a certain region cannot be transposed to another region without appropriate modifications. Because of the rarity of very severe tropical cyclones, coupled with the scarcity of observations in the intense part of the storms, the physical characteristics of cyclones in different regions are not completely known, and these uncertainties should be taken into account in the modelling.

Probable maximum tropical cyclone

5.34. For the purposes of the application of the methods discussed in this Safety Guide, a PMTC is a hypothetical steady state tropical cyclone having a combination

of values for meteorological parameters chosen to give the highest sustained wind speed that can reasonably occur at a specified coastal location. From the values of the meteorological parameters a PMTC should be derived and used to compute the maximum surge at coastal points, on the assumption that the PMTC approaches along the most critical track.

5.35. The methods for evaluating the PMTC are still undergoing development so that caution should be exercised in carrying out the evaluation. In this regard, modern techniques of determining some of the tropical cyclone parameters on the basis of observations made by aircraft and satellites have experienced a significant evolution and should be considered for application.

Data for design purposes

5.36. The maximum credible wind speed at the site should be specified. This value should be compatible with those resulting from available data recorded at the site or at nearby stations. Likewise, other features of interest for design, such as the vertical profile of the wind velocity or the duration of the wind intensity above specified levels, wind borne projectiles or surges should also be described.

LIGHTNING

5.37. Lightning transients exhibit extremely high voltages, currents and current rise rates. Damage is usually categorized as either direct or induced (indirect). The extreme electric field created under certain circumstances produces point discharges and can cause breakdown (a conductive path) in all but the most robust of insulators.

Once a path has been established for the return stroke, currents of tens to hundreds of kiloamperes flow.

5.38. While it is impossible to predict exactly when and where lightning will strike, statistical information collected over the years can provide some indication of the areas with the highest probability of lightning activity as well as the seasons and times of day when such activity is most likely to occur. It should be noted that lightning is an unpredictable transient phenomenon with characteristics that vary widely from flash to flash and whose measurement is difficult.

5.39. A commonly used method of presenting data on the occurrence of lightning is the isokeraunic map. Contour lines depict the number of thunderstorm days per month or year that a particular region can expect to experience. These maps are based on weather service records over an extended period of time (30 years for example). A thunderstorm

day is defined as any day during which a trained observer hears thunder at least once.

Although these maps are regularly referred to by persons who perform risk analyses for structures and systems that are vulnerable to lightning, they are a poor indicator of actual lightning activity. This is because one thunderstorm day will be noted whether a single thunderclap or 100 are heard on that particular day. In addition, recent studies indicate that thunder was not heard for 20–40% of lightning flashes detected.

5.40. While the probability of lightning striking in a particular area is often evaluated from statistically determined values from isokeraunic map data based on thunderstorm days, such calculations should be viewed with caution. Despite this caveat concerning the use of isokeraunic maps of thunderstorm days, they may be

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